This invention relates to a method for checking the state of an apparatus for testing sheet material having at least one illumination device, transparent measuring window and detector.
An apparatus of the abovementioned kind is known e.g. from WO 98/26276. Light emitted by an illumination device impinges on sheet material to be tested through a transparent measuring window. The light diffusely reflected by the sheet material is then detected and evaluated.
The known test apparatus for sheet material involves the problem that the measuring window can soil during operation. When the measuring window is soiled, less light from the irradiation device passes to the sheet material to be tested, for example, thereby reducing the measuring signal. During testing of the sheet material it cannot be ascertained whether the weaker measuring signal is to be attributed to a soiled measuring window or to the state of the sheet material to be tested. This means that soiling of the measuring window can result in misjudgment of the sheet material to be tested. This impairs the reliability of the sheet material testing.
The invention is based on the problem of proposing a method and apparatus for checking the state of a test apparatus for sheet material which makes it possible to ascertain with little technical effort the soiling impairing measurement in a measuring window of the test apparatus. The problem is solved according to the invention by the features stated in the independent claims.
For ascertaining the soiling in a measuring window according to the invention the latter is irradiated with UV light by an illumination device, the UV light transmitted through the measuring window being filtered by a UV filter such that only a small fraction passes the filter and reaches a detector. This fraction of UV light transmitted through the filter is detected and evaluated for determining the degree of soiling of the measuring window.
By reducing the transmitted light by using a filter after the light has passed through the measuring window, there is an improvement in determining the degree of soiling of the measuring window over the prior art. Therefore, the use of a filter has the advantage that the determination of the degree of soiling of the measuring window can be carried out more precisely.
Preferably, evaluation is effected such that an illumination profile is recorded and stored for a clean measuring window and compared with the particular illumination profile currently obtained from the detected signal. The degree of soiling of the measuring window can be determined with reference to the deviation of the currently recorded illumination profile from the stored illumination profile. Instead of illumination profiles, one can also compare the integrated measured values with each other. In both cases a deviation indicates soiling of the measuring window.
For determining the degree of soiling one preferably increases the sensitivity of the detector for example by prolonging the integration time or the light intensity of the illumination device or the signal gain compared to normal operation during testing of the sheet material. Further advantages and developments of the invention result from the dependent claims and the following description and examples of the invention with reference to the enclosed figures, in which: